Could anyone explain this to me

Question

I don`t understand the two code that have bolded and how come he need to subtract ten . Could anyone rewrite this two code in more easier way.
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.*;
import java.lang.*;
public class ReadFlights
  public static void main( String [] args )
    char[] refAlpha = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
    int[] amount = new int[26];  
     try {
      FileReader fr = new FileReader( "input.txt" );
      BufferedReader br = new BufferedReader(fr);
      int c;
      while  ( (c = br.read()) != -1)  // end of the file?
       char upperC = Character.toUpperCase((char)c);
int a = Character.getNumericValue((char)upperC)-10;         if(a >= 0 && a <=25)
         amount[a] = amount[a] + 1;
// loop for printing out the values of the counting array
   for(int i=0; i < amount.length; i++)
       System.out.println(  refAlpha[i] +": " + amount[i] + "\n" );
       fr.close();
catch(FileNotFoundException e)
  System.out.println("File not found");
  catch(IOException e)
}

Answer 1

You need to read the API documentation to find out what getNumericValue() is returning.
Quoting from there:
The letters A-Z in their uppercase ('\u0041' through '\u005A'), lowercase ('\u0061' through '\u007A'), and full width variant ('\uFF21' through '\uFF3A' and '\uFF41' through '\uFF5A') forms have numeric values from 10 through 35.